I
JOHN N. ROMINE Phillips Petroleum Co., Bartlesville, Okla.
Standards for Laboratory Sample Cylinders This guide for pressure containers ensures better accident protection in laboratories and pilot plants
T o
PREVENT Loss of samples, protect personnel, and reduce hazards from fire or escape of toxic chemicals, sample cylinders used in laboratories and pilot plants must meet safety standards. Before such standards can be established, however, size of samples and pressures under which they are used or stored must be known; also chemical properties are important to determine if cylinder attack occurs with resulting damage or sample contamination. I n these laboratories and pilot plants, the majority of samples used range from 10 to 4000,ml. with pressures up to 1800 pounds per square inch. Eight sizes of cylinder are adequate, and aluminum, carbon steel, and Type 304 or 316 stainless steel are generally satisfactory materials of construction. Aluminum is particularly desirable because of its light weight. For the failures of aluminum cylinders illustrated, liquefied gas samples were introduced at reduced temperature; containers were liquid full when ambient temperature was reached. After volume requirements are determined, standards must be set up for pressures under conditions where the
cylinders are used. Two useful sets of such standards have been designed-one by the American Society of Mechanical Engineers and the other by the Interstate Commerce Commission. The ASME code of which Section VI11 deals with unfired boiler and pressure vessels is intended for industrial application and exempts most sample cylinders because their inside diameter is less than G inches. However, its minimum construction requirements for design, fabrication, and inspection are as reliable as any other guide atailable. This ASME code defines maximum allowable pressure as “the highest internal pressure permissible in a vessel as determined by the design formulas for the nominal thicknesses of all parts to be considered, and which shall be used as the basis for determining the test pressure a t test temperature.” Type A-4 breathihg-oxygen cylinders have come into laboratory use as sample containers and pressure vessels since World War 11. They are war surplus and readily available a t low cost. Their thin wall construction make them light in weight and easy to handle. These breathing-oxygen cylinders are
nonmagnetic and according to chemical analyses, were probably Type 302 or 304 stainless steel. Because the ASME code does not list stress values for Type 302, 304 which is assigned to the 300 series, was used. Also, this code does not consider the type of reinforcement shown on cylinder B. Some breathing-oxygen cylinders are made of carbon steel. These containers are stenciled to indicate the volume of oxygen available when internal pressure is reduced from 400 to 50 pounds per square inch which in turn indicates a maximum allowable pressure of a t least 400 pounds. But 1956 ASME design formulas for calculating maximum allowable pressures show much lower values. Data for these cylinders (illustrated) are fitted into ASME formulas (Table I). Using these data, maximum allowable pressure for these breathing-oxygen cylinders is calculated (Table I). This method gives one cylinder a rating of 260 pounds per square inch and the other 193. Other cylinders have been checked and calculated to a maximum allowable pressure of 150 pounds per square inch. Therefore, 150 pounds is recommended for this type of cylinder VOL. 49, NO. 10
OCTOBER 1957
1747
These aluminum cylinders were used with excessive pressure A. 6.
Ductility of the metal caused deformation Rather than bulging, the shell ruptured i n a brittle-type fracture
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Table I.
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in general service, unless special equipment for measuring wall thickn-ss and manpoiver is available to check each cylinder. This adds to the low initial cost---rating must be identified with each cylinder and stenciling these data on the thin all may cause damage. Painting may not be satisfzctory; silvcr soldering or brazing a metal plate containing the information to the cylinders can be considered. Oxygen is essentially noncorrosive to metals from Tvhich these cylinders are fabricated; therefore, it should have no eflect on the maximum allowable pressure rating. However, components present in samples from cylinders used in general laboratory and pilot plant service could cause invisible corrosion which would affect wall construction and l o ~ e maximum r allo\vable pressure. Intergranular corrosion is one kind of attack to be expected in heat-affected
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ASME Calculations of Maximum Allowable Pressure for Breathing-Oxygen Cylinders Cylinder
Cylinder
B
A For hemispherical head, thickness and P not exceeding 0.356L and 0.66.5SE, respectively P = 2 SEt/L
+ 0.2t
Where P = design or maximum allowable working pressure S = ASME maximum allowable stress for 304 stainless steel, temp. -20' to 100' F.
S = 18,750 lb./sq. in.
S
E = lowest efficiency of any joint in the head. For seamless heads, use E = 1
E = l
E = l
t = minimum thickness of head after forming, exclusive of corrosion allowance
t
L = inside spherical or crown radius
L
= 0.036
=
= 18,750 lb./sq.
in.
1 = 0.0256 in.
in.
I, = 2.1744 in.
2.589 in.
For cylindrical shells, where thickness and P does not exceed Y, of inside radius, or 0.385 SE, respectively 18,750 X 1 X 0.0256 = 1931b./sq.inb 2.4744 - 0.6 X 0.0256
p = -
s = 15,750 !b.'sg. in.
s = ASME maximum allowable stress for 304 stainless steel, temp. for - 20' to 100" F.
s = 18,750 lb./sq.
E = Efficiency of longitudinal seamless cylinder shell
E = l
E = l
t = minimum thickness of shell plates, exclusive of corrosion allowance
t = 0.036 in.
t =
R
R
R = 2.4744 in.
joint
in
= inside radius of shell, before corrosion
= 2.589 in.
allowance is added
1748
INDUSTRIAL A N D ENGINEERING CHEMISTRY
in.
0.0256 in.
SAFETY I N CHEMICAL INDUSTRY
A
B
C
D
A Choice of poor construction material caused these leaks
4 The end was pushed out of this aluminum bomb faulty design
zones of welds in Type 302 or 304 stainless steel cylinders. Two other kinds possible for these cylinders or those made from carbon steel are stress corrosion cracking and chemical attack on the interior wall. If this type of breathing-oxygen cylinder is used for laboratory samples, education and control on their limitations need more consideration than for cylinders meeting ASME code requirements. I n hydrostatic tests to failure, it is interesting that a cylinder withstood more pressure without than with reinforcing bands. To select satisfactory materials of construction, the designer must know the nature, concentration, and effect of the chemicals on various metals used in sample cylinders. Sample-cylinder suppliers knowledge of metallurgy may be limited; therefore, consultation with a metallurgist or corrosion engineer would be more helpful. Frequently, too much is expected of stainless steels. For example, they resist corrosion under general oxidizing conditions where carbon steel is attacked, but they are attacked by halogens and their salts. Also, limitations of materials need to be understood by the laboratory and pilot plant personnel. Contamination by metal in the container must also be considered. To meet ASME code requirements,
because of
designs must specify all materials of construction. Fabrication and materials require checking and the vessel must be tested. For the vessel or sample cylinder larger than 6 inches in diameter to carry the ASME code stamp, ratings must be certified by a qualified inspector. T h e design must follow approved prac-
Cylinders A and C split; B and C developed pinholes; and D cracked
tices which limits deviations and results in equipment satisfactory for laboratory and pilot plant service. I t is recommended that industrial concerns with their own design and fabrication shop follow ASME code or have it followed by their supplier. With these standards as guides, specified or equivalent materials must be used. Many types of carbon and stainless steel-eg., free machining grades of the AIS1 1100 series and Type 303 stainless, have properties which limit
These are typical breathing-oxygen cylinders which since World W a r II have come into laboratory use as sample containers and pressure vessels VOL. 49, NO. 10
OCTOBER 1957
1749
In hydrostatic tests to failure, breathing-oxygen cylinders withstood more pressure without than with reinforcing bands
their use in parts of sample cylinders. They have a relatively high allowable percentage of sulfur or phosphorus which gives poor ductility and in case of failure can result in flying steel fragments. Also, although threads are easily cut on free machining steel, they are notch sensitive and can fail from either stress fractures a t their roots or marking impressed on the surface. Thickness of metal in this area will help but failure can still occur. Such materials, normally availabIe in the shop, should not be used in any part of the sample cylinder subjected to tensile stresses. Cylinders meeting ASME code requirements are satisfactory for interlaboratory and pilot plant use, but those shipped by commonn carrier must be approved by the Interstate Commerce Commission. I n some phases of design and rating, ICC regulations are more restrictive than the ASME code-e.g., specifications for steel cylinders which prohibit welding or brazing for any purpose except attaching a nonpressure part such as neckrings and footings only to tops and bottoms of cylinders having a service pressure of 500 pounds per square inch or less. A more intricate test method for satisfactory stress determination requires that cylinder expansion from hydrostatic test pressure be measured by water displacement. Materials of construction are more limited. These increased restrictions are imposed because handling and exposures not normally existing around a laboratory or pilot plant may be encountered. This type of cylinder should be available for shipments across state lines. Another point in design, which the previously noted standards do not consider, affects draining and cleaning. All inside corners should have adequate radii to eliminate stress points which can start from right angle corners. Cost is important in developing stand-
1750
These cylinders meet ASME and ICC standards but the projecting threaded necks of the middle cylinder makes cleaning and draining difficult
ards. Table I1 compares 1356 costs from a national supplier furnishing Type 304 stainless steel cylinders according to ICC standards with local prices for the same items built to ASME code requirements. ICC cylinders are cheaper in Bartlesville, Okla., but delivery for ASME cylinders is faster. Also, cylinders having a capacity up to 1000 mi. and a rating of 1800 pounds per square inch cost no more than those having a rating of 400 pounds. There are other considerations in developing standards. After a vessel is built to withstand higher than the design pressure, it should be given the higher maximum allowable pressure. Some manufacturers stamp on the cylinder the test pressure, and as a result the cylinder may be used at this pressure in the laboratory or pilot plant. I n this company, thercfore, manufacturers are requested to indicate only the maximum alloivable pressure. Cylinders should be checked periodically to ensure that assigned ratings are satisfactory. I t may be desirable to design sample cylinders having wrench flats; thus, a wrench that
\vi11 not damage the cylinder with teeth marks may be used when a strap Tvrench is unavailable. Conclusions
A survey should be done to determine the minimum number of cylinder types needed. Thus, less stock is needed, and users will have a part in the program. Cylinders shipped by common carrier must. of course, conform to ICC specifications, but for those having restricted handling, the ASME code is recommended as a minimum requirement. M’hen ICC cylinders cost less than the ASME type, they are more desirable as a standard because the latter may be inadvertently used as shipping containers. Chemical effects of samples on metals must be considered in selecting materials of construction. The material should yield before failure to reduce flying fragments. Cylinder metal should not contaminate the sample.
Acknowledgmen+ The author wishes to thank members of the test section of the engineering department for their assistance.
Table
II.
1956 Prices for Single-Outlet Sample Cylinders
T’ol., MI.
10 10 75 75 300 300 500 500 1000
1000 1 gal. 1 gal.
INDUSTRIAL AND ENGINEERING CHEMISTRY
Press. Rating, Lb./ss. In.
400 1800 400 1800 400 1800 400 1800 400 1800 400 1800
Refere n ce s
Manufacturers’ Price National Local 96 9.25 9.25 10.75 10.75 16.75 16.75 18.50 18.50 34,OO 43.00 49.00 75.00
8
16.75 21.40 19.50 25.40 40.80 53.13
(1) American Society of Mechanical Engi-
neers, Sew York, ASME’ Boiler and Pressure Vessel Code Section VIII, “Rules for Construction of Unfired Pressure Vessels:’’ 1956. ( 2 ) Interstate Commerce Commission, Washington, D. C . , Tariff Regulation 9. RECEIVED for review .April 20, 1957 ACCEPTED July 18, 1957 Division of Industrial and Engineering Chemistry, High Pressure Symposium, 131st hleeting, ACS, Miami; Fla., April 1
A?“,
